In response to a question from POLITICO on board an RAF Voyager plane on her way to New York, the prime minister said she would be able to maintain frictionless trade and strike "good, improved trade deals" with countries around the rest of the world.

"Just think about this: If having those EU regulations stopped good trade deals, how come the EU has done trade deals with countries around the rest of the world?" May said.

Now she tells us?

David Henig, a former U.K. trade official who was involved in U.S.-EU trade negotiations, agreed that a shared rulebook with the EU could cause problems for the U.S. trade deal, although it may not be terminal.

"The U.S. agriculture people need to be able to export, they insist on their standards for export," he said.

Nick Allen, chief executive of the British Meat Processors Association, said the U.K. has particularly high animal welfare standards and very prescriptive hygiene standards, and anything that means they have to compete with lower standards is a problem for them.